1. Field of the Invention
The present invention relates to an annealing method by irradiation with light beams which is suited for use in various heat treatments, such as in a process for manufacturing semiconductor devices. More particularly, the invention is useful when applied to a short-time annealing of a semiconductor substrate which has been injected with ions, as it is well-controlled and capable of reproducibly activating the injected impurities and thereby reproducibly forming a good impurity region.
2. Description of the Prior Art
Recently, a lamp annealing method has been put into practice as a heat treatment method in a manufacturing process for semiconductor devices. In this prior radiation annealing method, a material, or a sample, is heated by radiation with light beams from a lamp light source and the method has a feature that its use makes it possible to raise the temperature of the sample directly and quickly. In practicing this method, it has been desired that the sample be heat-insulated from the sample holder. Therefore, the sample holder is made of a material with low heat conductivity and, further, it is designed so that its sample holding area is as small as possible. In the measurement and control of the temperature of the sample, there have so far been two methods in use.
The first method is arranged as shown in FIG. 1, such that a sample 5 to be annealed (a semiconductor wafer, for example) and a sample as a temperature monitor 6 are mounted on sample holders 3, 4, respectively, which are disposed between lamp light sources 1, 2 connected to a specific power source (not shown). A thermocouple 7 is disposed in contact with the back side of the temperature monitor sample 6, and measurement of the temperature of the sample 5 is indirectly made by means of the output of the thermocouple 7, and control of the temperature of the sample 5 is made by feeding back the output of the thermocouple 7 to the power source of the lamp light sources 1, 2.
In the second method, the relationship between the temperature rising characteristic of the sample 5 and the power source voltage is obtained in advance and the temperature control of the sample 5 is made by applying the lamp light sources 1, 2 with specific voltage corresponding to a desired temperature.
However, there were some defects in the sample holding arrangement as shown in FIG, 1. That is, although it was designated that the holding area of the sample would become very small, the heat capacity of, and heat conduction in, the holding member are not negligibly small in a quickly heating and cooling temperature cycle. Accordingly, it was unavoidable that temperature differences were produced between various portions of the sample. As a result, when the above described method, whether the first method or the second method, was used, not only was uneven annealing conducted within the sample, but also, in the worst case, crystal defects resulting from thermal stress were produced. Specifically in the first method, there is a time lag as large as about 10 seconds between the output of the power source of the lamp light sources 1, 2 and that of the thermocouple 7, and therefore, in an annealing process that should be finished within 10 to 20 seconds, for example, practical temperature control could not be exercised satisfactorily.
Further, in the second method, since there was no feedback provided to the power source of the lamp light sources 1, 2, it was not possible to compensate for variations in the power source or deterioration in the lamps, and so, the method was defective in that annealing could not be reliably and precisely repeated.
On the other hand, a method of sample holding ensuring heat insulation between the sample and sample holder, is known in the form of gas flow suspension of the sample as shown in FIG. 2. In this method, a predetermined gas flow is introduced into a floating type sample holder 8 through one end and the gas is blown out from a number of orifices 8a provided in the holder whereby a sample 5 is made to be suspended by the gas under this condition, and the sample is annealed by a lamp light source (not shown). With this method, an improvement in the uniform temperature distribution within the sample may be achieved, but there is no other method for controlling the sample temperature than the previously described second method, and so, again, annealing cannot be conducted with satisfactory reproducibility. Further, the temperature rising and falling speeds cannot be freely regulated solely by the method of controlling the output of the power source of the lamp light source 1, 2, without monitoring the sample temperature.